The effect of strain of Holstein-Friesian cow on size of ovarian structures, periovulatory circulating steroid concentrations, and embryo quality following superovulation.

When managed under grass-based systems of production, the New Zealand (NZ) strain of Holstein-Friesian cow has superior reproductive performance compared to the North American (NA) strain despite having similar solids-corrected milk (SCM) yields. This study compared the ontogeny of early pregnancy events in NZ and NA cows. Ten NZ and 10 NA cows were submitted to a superovulation protocol on three occasions. Blood samples were collected daily from every cow from days -3 to +7 relative to a synchronized oestrus during each superovulation protocol. Pre-ovulatory oestradiol concentrations, follicle diameter, post-ovulatory progesterone concentrations, corpus luteum (CL) diameter, and circulating insulin-like growth factor-I concentrations did not differ between the two strains. Uteri were non-surgically flushed 7 days post-AI, embryos were isolated and graded. The proportion of transferable embryos recovered was higher (P<0.01) in the NZ cows compared with the NA cows. A greater (P=0.01) proportion of the recovered structures were at the blastocyst stage in the NZ cows. Peak SCM yield and body condition score (BCS) at the time of peak SCM yield were not different between strains. However, during the experimental period the NA cows maintained significantly higher daily SCM yields, whereas the NZ cows replenished significantly greater levels of BCS. The results indicate that differences in periovulatory steroid concentrations and size of ovarian structures do not explain the differences in embryo quality between the two strains. However, strain differences in nutrient partitioning from the time of peak SCM yield through late lactation may provide the key signals responsible for superior embryo quality in NZ cows.